1. Field of the Invention
This invention relates to an improved process and device for monitoring time-temperature histories of perishable articles wherein an inactive form of a diacetylene compound, containing at least one --C.tbd.C--C.tbd.C group, is contacted with an activating vapor to convert it to an active form, capable of undergoing a color change upon thermal annealing.
2. Brief Description of the Prior Art
Time-temperature history indicators are known in the art which incorporate diacetylene compounds as the active indicator and are described in U.S. Pat. No. 3,999,946 (Patel et al. to Allied Chemical, 1976), the subject matter of which is hereby incorporated by reference.
U.S. Pat. No. 3,501,297 discloses a process for producing visual photographic images wherein a colorless mixture of diacetylenes is exposed to ultraviolet radiation, thereby being converted to a deep blue/bronze color, and then transforming the resulting mixture to a red product by exposing to warm ethanol vapors for about 5 minutes. However, such a process does not suggest a device in which an inactive form of diacetylene can be converted to an active form of diacetylene by contacting with a vapor.
U.S. Application Ser. No. 854,933 (Patel and Yee, to Allied Chemical, 1977), the subject matter of which is hereby incorporated by reference, discloses a device in which an inactive diacetylene is converted by melt or solvent recrystallization to an active form, capable of exhibiting a color response upon thermal annealing. However, the device requires contacting inactive diacetylene compound with liquid solvent for the activation process, said quantity of solvent representing an economically unattractive aspect of activating such devices.
The references Zeit, fur Naturforschung V. 24b, pp. 824-832 (1969) by G. Wegner, and Acta Cryst. (1978) B34, 143-147, by J. J. Mayerle and T. C. Clarke, disclose that the bis(p-chlorophenylurethane) of 2,4-hexadiyn-1,6-diol is relatively inactive toward photopolymerization and thermally induced polymerization.
U.S. Pat. No. 3,981,683 (1976) describes a sterility indicator comprising a backing strip of a dimensinally stable material, e.g. aluminum foil, having mounted thereon an organic compound, containing oxygen or nitrogen, in intimate contact with a wicking means and a cover strip bonded to the backing strip overlaying the organic compound and wicking means. The cover strip is a polymeric rate controlling film which permits water vapor to pass through at a rate sufficient to make the device operative at a temperature to be monitored. However, the use of inactive forms of diacetylenes as indicating materials is not suggested or mentioned.
U.S. Pat. No. 4,042,336 (1977) describes a device for measuring the time-temperature history of a product in which the history is displayed as a color front on an indicator, the distance of front advancement being a function of the temperature-time integral. However, the use of inactive forms of diacetylenes as indicating materials, which can be converted to active forms upon contacting an activating vapor, is not suggested or mentioned.
We have unexpectedly found that an inactive form of a diacetylene compound, containing at least one --C.tbd.C--C.tbd.C--group, can be activated prior to use, by allowing the inactive form to be contacted with a vapor, thus converting the inactive form to an active form, capable of undergoing a color change upon thermal annealing.